1. Statement of the Technical Field
The invention concerns antenna systems, and more particularly ring focus antennas configured for concurrent multi-band operation.
2. Description of the Related Art
It is desirable for microwave satellite communication antennas to have the ability to operate on multiple frequency bands. Upgrading existing equipment to such dual band capability without substantially changing antenna packaging constraints can be challenging. For example, there can be existing radomes that impose spatial limitations and constraints on the size of the reflector dish. The existing antenna location and packaging can also limit the dimensions of the antenna feed system. For example, the existing radome can limit the forward placement of the feedhorn and the subreflector. Similarly, modifications to the existing opening in the main reflector are preferably avoided. As a result, for small aperture reflectors, the feed horn and the subreflector must fit in a relatively small cylinder.
In view of these spatial limitations, special techniques must be used to maintain antenna efficiency. U.S. Pat. No. 6,211,834 B1 to Durham et al. (hereinafter Durham), concerns a multi-band shaped ring focus antenna. In Durham, a pair of interchangeable, diversely shaped close proximity-coupled sub-reflector-feed pairs are used for operation at respectively different spectral frequency bands. Swapping out the subreflector/feed pairs changes the operational band of the antenna. Advantage is gained by placement of the shaped subreflector in close proximity to the feed horn. This reduces the necessary diameter of the main shaped reflector relative to a conventional dual reflector antenna of the conventional Cassegrain or Gregorian variety. The foregoing arrangement of the feed horn in close proximity to the sub-reflector is referred to as a coupled configuration.
The coupled configuration described in Durham generally involves subreflector to feed horn spacing on the order of two wavelengths or less. This is in marked contrast to the more conventional sub-reflector to feed horn spacing used in a decoupled configuration that is typically on the order of several to tens of wavelengths.
Although Durham demonstrates how a ring focus antenna may operate at different spectral bands, sub-reflector-feed pairs must be swapped each time the operational band of the antenna is to be changed. Accordingly, that system does not offer concurrent operation on spectrally offset frequency bands.
U.S. Pat. No. 5,907,309 to Anderson et al. and U.S. Pat. No. 6,323,819 to Ergene each disclose dual band multimode coaxial antenna feeds that have an inner and outer coaxial waveguide sections. However, neither of these systems solve the problem associated with implementing dual band reflector antennas in very compact antenna packaging configurations.
A compact multi-band antenna system includes a main reflector having a shaped surface of revolution about a boresight axis of the antenna. The main reflector is operable at a plurality of frequency bands spectrally offset from each other. For example, the higher one of the frequency bands can be Ka-band and the lower one of the frequency bands can be X-band.
A multi-band feed system provided for the main reflector includes a shaped non-linear surface of revolution about the boresight axis of the antenna. A plurality of feed elements are also provided. A first one of the feed elements for a high frequency band is installed at a first feed element location separated by a first gap from a vertex of the shaped non-linear surface of revolution on the boresight axis of the antenna. For example, the first gap can be more than about four wavelengths at a frequency defined within the first one of the frequency bands from the vertex to the feed aperture.
The first feed element can be decoupled from the shaped non-linear surface of revolution and illuminates the shaped non-linear surface of revolution. The shaped non-linear surface of revolution functions as a subreflector for the first feed element. The subreflector defines a ring-shaped focal region about the boresight axis for illuminating the main reflector at a first one of the frequency bands.
A second one of the feed elements for a lower frequency band can be installed at a second feed element location separated from the vertex on the boresight axis by a second gap. For example the second gap can be less than about two wavelengths from the vertex of the shaped non-linear surface of revolution at a frequency defined within the second one of the frequency bands. Consequently, the second feed element is closely coupled to the shaped non-linear surface of revolution at a second one of the frequency bands.
The second feed element and the shaped non-linear surface of revolution can together form a single integrated coupled feed. The diameter of the focal ring of the main reflector at the lower frequency band is advantageously selected to be about the same size as the diameter of the shaped non-linear surface of revolution. Consequently, it is possible to use the single coupled feed to form a focal ring matched to the main reflector at the lower one of the frequency bands. In effect, the shaped non-linear surface of revolution in the single coupled feed performs as a splash plate. The single coupled feed also provides a transition from a circular to radial waveguide mode.
Notably, the single structure defining the shaped non-linear surface of revolution performs two very different functions at the two separate frequency bands. At the high band it functions as a sub-reflector whereas at the low band it functions as a splash plate defining part of the single coupled feed. In order to facilitate this result, the main reflector and the shaped non-linear surface of revolution can each have no continuous surface portion thereof shaped as a regular conical surface of revolution. Instead, these shapes can be numerically defined using computer modeling programs.
The invention can also include a method for operating a compact multi-band antenna system. The method can include the steps of providing a main reflector having a shaped surface of revolution about a boresight axis of the antenna, forming a ring-shaped focal region about the boresight axis, and using a subreflector in the far field relative to a first feed element aligned with the boresight axis. Further, a second feed element can be aligned with the boresight axis in a nearfield position coupled to the sub-reflector to form in combination with the sub-reflector a single feed that transforms a circular waveguide mode into a radial waveguide mode for illuminating the main reflector. The first feed element can be selected to operate at a relatively higher band as compared to the second feed element. For example, the first feed element can operate within Ka-band and the second feed element can operate within X-band.
According to another aspect of the invention, the method can include positioning an aperture of the first feed element spaced more than about four wavelengths from a vertex of the shaped non-linear surface of revolution at a frequency within the first spectrally offset frequency band, and positioning an aperture of the second feed element spaced less than about two wavelengths from a vertex of the shaped non-linear surface of revolution at a frequency within the second spectrally offset frequency band. A focal ring of the main reflector can be advantageously selected to be about the same size as the shaped non-linear surface of revolution. The method can also include selecting the main reflector and the subreflector to have no continuous surface portion thereof shaped as a regular conical surface of revolution.
According to another aspect, the invention can include a method for feeding a compact main reflector of an RF antenna on a plurality of spectrally offset frequency bands. The method can include the steps of forming a focal ring for a main reflector by positioning an RF source at a first frequency within a first frequency band in the far field relative to a shaped non-linear surface of revolution so that the shaped non-linear surface of revolution operates as a subreflector. A second focal ring can be formed for the main reflector by positioning a second RF source in the nearfield of the shaped non-linear surface of revolution. The second RF source can interact with the shaped non-linear surface of revolution to form a single feed network at the second RF frequency. The single feed network can form a coupled feed focal ring for the main antenna where the single feed network transforms a circular waveguide mode of said second RF source to a radial waveguide mode.